In order to save power of the UE and prolong its standby time, a Discontinuous Receive (DRX) mechanism is introduced, which only requires to discontinuously monitor the control channel of Evolved Node B (eNB) instead of a continuous monitoring when the UE is in a connection state.
FIG. 1 schematically shows the principle of DRX in a Long Term Evolution (LTE) system of the prior art. Here, the term “On Duration” represents a time period in which the UE monitors the control channel, during which the radio frequency channel of the UE is open and the control channel is monitored continuously. During the rest of the time period, the UE is in a Sleep state and the radio frequency link of which will be shut down so as to save power. Usually, the “On Duration” is periodical, and its cycle is configured by the eNB. In order to avoid a large time delay of communication between the eNB and the UE during the power saving of the UE, the concepts “long cycle” and “short cycle” are both introduced. In the short cycle, “On Duration” appears more frequently than in the long cycle. The long cycle and the short cycle can be configured simultaneously to shorten the time during with the UE monitors the control channel, thereby reducing the time delay during data transmission.
In order to perform DRX operation, a plurality of Timers are designed for the LTE, and an operation process in the DRX state is given in combination with a Hybrid Automatic Request (HARQ) process, the related Timers include:
1. Inactivity Timer: this Timer is started when the UE receives a control signal transmitted initially by the HARQ during the “On Duration”. Before this Timer expires, the UE continuously monitors the control channel. If the UE receives the control signal transmitted initially by the HARQ before the inactivity Timer expires, then the inactivity Timer will be stopped and restarted.
2. RTT Timer: it is only suitable for downlink (DL). If the UE receives the control signal retransmitted by the HARQ, this Timer is started. If data corresponding to the HARQ process has not been decoded successfully after the previous transmission of the HARQ, the UE is started a retransmission Timer after the RTT Timer expires. If data corresponding to the HARQ process are decoded successfully after the previous transmission of the HARQ, the UE does not start the retransmission Timer after the RTT Timer expires.
3. Retransmission Timer: during the retransmission Timer is started, the UE monitors the control channel and is waiting for the retransmission corresponding to the HARQ process.
FIG. 2 schematically shows each Timer's action during the DRX process in the prior art. As shown in FIG. 2, firstly On duration Timer is running, during which the eNB schedules the initial transmission of the DL at the time t1, so the Inactivity Timer is running and the HARQ RTT Timer is started. At the time t2, the Inactivity Timer expires. At the time t3, the HARQ RTT Timer expires. As the initial transmission at the time t1 is not successful (the UE feeds NACK back), the Retransmission Timer is started. At the time t4, the eNB schedules the first retransmission, thus the Retransmission Timer is stopped and the RTT Timer is started. At the time t6, the RTT Timer expires, and the first retransmission at the time t4 is still not successful (the UE feeds NACK back), thus the Retransmission Timer is started. At the time t7, the eNB schedules the second retransmission, thus the Retransmission Timer is stopped and the RTT Timer is started. As the second retransmission is successful (the UE feeds ACK back), the Retransmission Timer will not be started after the RTT Timer expires.
It can be known from FIG. 2 and the above process that the RTT Timer can increase the time during which the UE is in the sleep state, such as the time period T1 and T2 in FIG. 2. The RTT Timer could be set to control the UE is in the sleep state between two transmissions by setting, so that the UE enter the monitoring state at a suitable time.
The disadvantages of the prior art lie in: in the current LTE system, the setting time for the RTT Timer is 8 ms, which equals to the minimum transmission time interval of the Frequency Division Duplex (FDD) system. As eNB can only schedule the retransmission after this 8 ms minimum transmission time interval, the 8 ms RTT Timer is a relatively optimized design for the FDD system. But for the Time Division Duplex (TDD) system, there is a problem. As the minimum downlink transmission time interval of the TDD system is related to both UL/DL configuration and the sub-frame number of the downlink sub-frame, it is not always applicable for the TDD system to set the RTT Timer to be 8 ms, which will increase the power consumption of UT and not be disadvantageous to the power saving performance of UE.